The present invention relates to an optical communication system for allowing two remote stations to exchange information carrying light with or without the aid of a repeater. The two remote stations may be two fixed stations, or a fixed station and a mobile station, or even two mobile stations such as vessels.
In an optical communication system, a first station sends out information carrying light, or an optical signal, to a remote second station with or without a repeater employed depending upon the distance between the stations as well as other conditions. The two remote stations are often constructed on the rooftops of buildings. In such a case and, particularly when the buildings are tall, the system suffers from the drawback that the accuracy of communications is lowered due to displacements of the buildings which result from, for example, a difference in temperature between building portions in the sun and those out of the sun. That is, the transmitting station would emit an optical signal in a direction other than proper one while the receiving station would misorient the light receiving surfaces of lenses, disenabling the receiving station to catch the signal or reducing the quantity of received light.
Assume that the transmitting and receiving stations are remote from each other by a distance of two kilometers, and that a light source at the transmitting station has a diameter of three centimeters by way of example. Then, light emanating from the light source spreads itself to a diameter as large as about three meters when received by the receiving station and, therefore, the whole quantity of transmitted light cannot be received unless use is made of a lens having a diameter of three meters. As previously stated, any error in the relative position between the transmitter and the receiver would reduce the quantity of received light or practically make it zero. This constitutes another cause of a decrease in the quantity of received light and, thereby, that of a decrease in the communication accuracy, although a certain degree of redundancy is available concerning misorientation of the transmitter and receiver relative to each other.